The long term evolution (LTE) technology supports two kinds of duplex methods: frequency division duplex (FDD) and time division duplex (TDD). In a LTE system, transmission from a base station to a user equipment (UE) is called downlink and transmission from the UE to the base station is called uplink.
A FDD system includes two carriers, called uplink carrier and downlink carrier respectively, in which the uplink carrier is used for uplink transmission and the downlink carrier is used for downlink transmission. The uplink carrier and the downlink carrier have a same system frame structure consisting of 10 uplink sub-frames or downlink sub-frames each of which has a time length of 1 millisecond (ms), as shown in FIG. 1.
FIG. 2 is a schematic diagram illustrating a frame structure of a LTE TDD system. The length of each radio frame is 10 ms, and each radio frame is equally divided into two half-frames with a length of 5 ms each. Each half-frame contains 8 time slots with a length of 0.5 ms and 3 special fields. The 3 special fields have a total length of 1 ms. The 3 special fields are respectively downlink pilot time slot (DwPTS), guard period (GP), and uplink pilot time slot (UpPTS). Each sub-frame consists of two consecutive time slots. Based on the frame structure shown in FIG. 2, 10 sub-frames are shared for uplink and downlink within every 10 ms, and each sub-frame is either configured for uplink or is configured for downlink. A sub-frame that is configured for uplink is called an uplink sub-frame, and a sub-frame that is configured for downlink is called a downlink sub-frame. In the TDD system, 7 uplink/downlink (UL/DL) configurations are supported. As shown in Table 1, D represents a downlink sub-frame, U represents an uplink sub-frame, and S represents a special sub-frame that contains 3 special fields.
TABLE 1ReferenceSwitch ConfigurationPointSub-frame IndexNumberPeriod012345678905msDSUUUDSUUU15msDSUUDDSUUD25msDSUDDDSUDD310msDSUUUDDDDD410msDSUUDDDDDD510msDSUDDDDDDD610msDSUUUDSUUD
The LTE system supports a hybrid automatic repeat request (HARQ) mechanism, and the basic principle of the HARQ mechanism is that a base station allocates uplink resources for a UE; the UE uses the uplink resources to send uplink data to the base station; and the base station receives the uplink data and sends HARQ indicator information to the UE, and the UE performs retransmission of the uplink data according to the indicator information. Specifically, the UE uses a physical uplink shared channel (PUSCH) to bear the uplink data, the base station uses a(n) (enhanced) physical downlink control channel ((E) PDCCH) to bear scheduling and control information of the PUSCH, i.e., uplink grant (UL Grant), and the base station uses a physical hybrid-ARQ indicator channel (PHICH) to bear the HARQ indicator information. In the above procedure, a timing position of a transmission and a timing position of a subsequent retransmission on the PUSCH are determined based on pre-configured timing relations which include a timing relation of uplink grant to PUSCH, a timing relation of PHICH to PUSCH, and a timing relation of PUSCH to PHICH. These three timing relations are all called synchronous HARQ timing relation of PUSCH in the following.
For the timing relation of uplink grant/PHICH to PUSCH, assuming that the UE receives uplink grant or PHICH in downlink sub-frame n (n being a sub-frame index number, similarly hereafter), then the uplink grant or the PHICH is used to control PUSCH in uplink sub-frame n+k. In a FDD system, the value of k is 4, and in a TDD system, the value of k is decided by uplink reference TDD uplink/downlink configurations (UL-reference TDD UL/DL configurations) of a cell, as shown in Table 2.
TABLE 2Reference Configuration Sub-frame Index nNumber012345678904,76,74,76,716464244344444454677775
For the timing relation of PUSCH to PHICH, when the UE receives PHICH in downlink sub-frame n, the PHICH indicates the HARQ-ACK information of PUSCH in uplink sub-frame n−h, where for the FDD system, the value of h is 4, and for the TDD system, the value of h is decided by the UL-reference TDD UL/DL configurations of the cell, as shown in Table 3.
TABLE 3ReferenceConfigurationSub-frame Index nNumber01234567890747414646266366646656664746
According to a duplex mode of the cell, if the duplex mode is TDD, then a synchronous HARQ timing relation of PUSCH adopted by the cell can be determined according to the reference UL/DL configurations of the cell, so as to realize synchronous transmission of PUSCH according to the synchronous HARQ timing relation of PUSCH.
As seen from the above the description, compared to the TDD system, the HARQ working mechanism of the FDD system is very simple, which is a big advantage of the FDD system. However, compared to the TDD system, there is a very obvious disadvantage in the FDD system. That is, UL/DL resource allocation is not that flexible. Because the TDD system may change the UL/DL resource allocation to make the ratio of uplink frames to downlink frames be adapted to the ratio of uplink services to downlink services, but the bandwidth of UL/DL carriers cannot be changed flexibly in the FDD system. With booming of mobile internet, the habits of mobile users have changed a lot. Generally, the amount of downlink services is much larger than the amount of uplink services in a cell, and in some circumstances, the ratio of uplink services to downlink services in a cell changes dynamically.
To adapt to the above change, flexible FDD technologies are paid more and more attention to. The flexible FDD technology may convert a part of uplink sub-frames on a FDD uplink carrier into downlink sub-frames, so as to flexibly change the ratio of uplink sub-frames to downlink sub-frames on the uplink carrier of a FDD system to adapt to the ratio of uplink services to downlink services of a current cell. It should be noted that currently sub-frames on an uplink carrier in a FDD system are all used for uplink transmission. That is, they are all uplink sub-frames. Since the flexible FDD technology introduces downlink sub-frames into uplink carriers, the technology may cause new problems for uplink transmission in a FDD system, e.g., how to realize transmission on PUSCH of a flexible FDD supported UE (FFUE), how to be compatible with uplink transmission of a backward UE, etc.
Obviously, the flexible FDD technology will cause new problems for the uplink transmission, but currently there is no solution to solve the problems yet.